The present invention relates to an electrohydraulic control arrangement for controlling a hydraulic drive means, such as a hydraulic cylinder in which each of the opposed sides of a double-acting working piston delimits a pressure chamber, with the arrangement including at least one control valve that is provided with a control slide unit that is spring-loaded at one end and can be shifted out of a rest position against the spring force, and with the arrangement also including a servomotor for controlling the control slide unit.
To form electrohydraulic control arrangements, directly controlled regulating valves are generally utilized; these valves can also be designated as electrohydraulic continuous valves. Such a regulating valve contains, in the valve housing, a spring-loaded control slide unit on which are disposed all of the leading edges that are required. These leading edges must be functionally balanced at manufacture; they cannot be subsequently adjusted relative to one another, not even partially. Due to the influence of a return spring, the control slide unit is held in a mechanically limited end position that at the same time represents the so-called "not-stop-function". From this end position, the control slide unit can be moved by the force of a solenoid (control magnet) into the various valve positions in order to attain the respectively desired hydraulic symbol. The extent of the stroke movement of the control magnet (theoretical value) is determined by a distance-measuring system. With a suitable control, a control circuit can be provided with such an arrangement.
With one such directly controlled regulating valve, the neutral position cannot be detected directly; rather, this neutral position can be determined only indirectly via the performance of the receiving device that is to be controlled. This is extremely unsatisfactory since during adjustment or activation of the regulating valve this leads to undesired and uncontrolled movements of the consuming device that is connected thereto.
A further drawback is that the so-called "not-stop-function" (regulating valve inactive) cannot be controlled separately; rather, this function is possible only by removal of the setting means. However, since the position of the control slide unit in the mechanical end position is not identical to the neutral position of the regulating valve (controlled condition), when the "not-stop-function" is established one or more symbol positions of the regulating valve are always passed over. This inherently leads to undesired and uncontrollable movements at the consuming device. Furthermore, impermissibly high delays occur at the consuming device upon disconnection from one of the possible regulating conditions having greater preset values; associated with these delays is a danger of damage in the hydraulic system. In order to counteract these negative effects, additional precautions have been adopted by installing a separate "not-stop-valve" in the pressure medium feed line and/or by installing safety pressure valves in the consuming device lines.
It an object of the present invention to obviate these drawbacks, and in particular to provide an electrohydraulic control arrangement with which the control valve can be activated independently of the respective presetting of the servomotor for controlling the control valve. In so doing, the setting of the control valve into the positions "active" or "inactive" should always have priority relative to the presetting value. In addition, the control arrangement should be such that upon reactivation of the arrangement, the presetting value can be adjusted independently of the position of the control valve, i.e. it should be possible to move the servomotor into the neutral position without any movements of the consuming device being connected therewith.